A number or series of numbers used to identify a given day with the least possible ambiguity. The date is usually expressed as the month, day of month, and year. However, integer numbers such as the Julian Date are also used to express the date.

Daylight Saving Time

The part of the year when clocks are advanced by one hour, effectively moving an hour of daylight from the morning to the evening. In 2007, the rules for Daylight Saving Time (DST) have changed for the first time since 1986. The new changes were enacted by the The Energy Policy Act of 2005, which extended the length of DST by about one month in the interest of reducing energy consumption. DST will now be in effect for 238 days, or about 65% of the year, although Congress retained the right to revert to the prior law should the change prove unpopular or if energy savings are not significant. Under the current rules, DST in the U.S. begins at 2:00 a.m. on the second Sunday of March and ends at 2:00 a.m. on the first Sunday of November.

Daylight Saving Time is not observed in Hawaii, American Samoa, Guam, Puerto Rico, the Virgin Islands, and the state of Arizona (not including the Navajo Indian Reservation, which does observe).

Daytime Protocol

A time code protocol used to distribute time over the Internet. The daytime protocol is described in the RFC-867 document, and is implemented by the NIST Internet Time Service.

Dead Time

The time that elapses between the end of one measurement and the start of the next measurement. This time interval is generally called dead time only if information is lost. For example, when making measurements with a time interval counter, the minimum amount of dead time is the elapsed time from when a stop pulse is received to the arrival of the next start pulse. If a counter is fast enough to measure every pulse (if it can sample at a rate of 1 kHz, for instance, and the input signals are at 100 Hz), we can say there is no dead time between measurements.

Disciplined Oscillator (DO)

An oscillator whose output frequency is continuously steered (often through the use of a phase locked loop) to agree with an external reference. For example, a GPS disciplined oscillator (GPSDO) usually consists of a quartz or rubidium oscillator whose output frequency is continuously steered to agree with signals broadcast by the GPS satellites.

Doppler Shift

The apparent change of frequency caused by the motion of the frequency source (transmitter) relative to the destination (receiver). If the distance between the transmitter and receiver is increasing the frequency apparently decreases. If the distance between the transmitter and receiver is decreasing, the frequency apparently increases. To illustrate this, listen to the sound of a train whistle as a train comes closer to you (the pitch gets higher), or as it moves further away (the pitch gets lower). As you do so, keep in mind that the frequency of the sound produced at the source has not changed.